For 104 prostate glands obtained at radical prostatectomy for adenocarcinoma, we mapped the tumor outline and determined the tumor volume, grade, and location relative to the transition zone boundary and location of the central zone. Among the 88 cancers whose probable zone of origin could be identified, 68% arose in the peripheral zone, 24% arose in the transition zone, and 8% arose in the central zone. Transition zone carcinomas had usually been diagnosed by transurethral resection (TUR) and often appeared to arise within BPH nodules; only two of 67 non-transition zone carcinomas had been diagnosed at TUR. Two-thirds of 21 transition zone cancers showed a distinctive histologic appearance; they were made up of columnar clear cells lining glands of widely variable size and contour. The transition zone boundary appeared to act as a barrier to the spread of non-transition zone carcinomas. We conclude that carcinoma typically arises in the region of the prostate that is susceptible to benign prostatic hyperplasia and that the great majority of Stage A (TUR) cancers are transition zone cancers. Non-transition zone cancers detectable at TUR are predominantly large tumors that are poorly differentiated and lack the clear cell histologic pattern.